Measurements of single- and double- differential σtt in the boosted topology, at $\sqrt{s}$ = 13 TeV, using the ATLAS detector

The large energy and luminosity reached by the Large Hadron Collider allow to perform differential measurements also in regions of the phase space where a massive particle, as the top quark, is produced with high transverse momentum ($p_T$). The analysis presented in this contribution is the differential measurement of $\sigma^{t\bar{t}}$, obtained considering events collected by the ATLAS experiment in 2015 and 2016 in the $e(\mu)$+jets channel, with at least one large-R jet with $p_T > 350$ GeV. The large-R jet mass is requested to be in the range [120,220] GeV and it is directly interpreted as the top quark decaying hadronically. The technique used to reconstruct the large-R (R=1.0) jet is the re-clustering of anti-$k_T$ jets with R=0.4. This method allows to forward the calibrations and uncertainties derived on the small-R jets directly on the large-R jet, reducing considerably the uncertainty on the differential measurement in the boosted region, with respect to previous publications. The method and dataset employed allowed to perform the first double differential measurement in the boosted regime. The events selected are corrected for the effects of the detector using an unfolding technique and the results are presented both in a fiducial phase space, defined to follow the detector-level selection, than extrapolated to a larger region of the phase space, limited only by the requirement of a top quark with $p_T > 350$ GeV.